Cathode ray tube



April 1939- F. H. NICOLL 2,153,269

CATHODE RAY TUBE Filed Feb. 25, 1956 L RI A 7' TORNEY Patented Apr. 4, 1939 UNITED STATES PATENT 0mm Signor to Electric and Musical Industries Lim ited, Hayes, Middlesex, England, a. company of Great Britain Application February 25, 1936, Serial No: 65,593 In Great Britain February 25, 1935 Claims. (01. 250 27) In the specification of patent application Serial No. 745,838, filed Sept. 28, 1934, Patent No. 2,119,119, dated May 31, 1938, there is described and claimed an electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope, in the order mentioned, a cathode, a first accelerator, a decelerator havingan apertured diaphragm, and a sec-f 0nd accelerator, the accelerators and decelerator being so disposed that the ray can pass through them to a screen associated with the tube, and the tube having means whereby the ray can be deflected over the screen, and electrostatic or electromagnetic means for focusing theray in a small spot upon the screen, characterised by means for applying to the accelerators potentials positive with respect to the cathode potential and to the decelerator a potential equal, or nearly equal, to the cathode potential, the shape and disposition of the electrodes and the potentials applied thereto being such that, in operation, increases of potential of the decelerator, in the negative sense with respect to the cathode potential, produce increases of current flowing to the first accelerator.

The said electric circuit as described and claimed in the patent application referred to may comprise a cathode ray tube having arranged within its envelope, between said second accelerator and said screen, a second decelerator adjacent said second accelerator, and a third ac-' celerator, characterised by means for applying to the accelerators potentials positive with respect to the cathode potential and to the decelerators potentials equal, or nearly equal, to the cathode potential. 7

With cathode ray tubes of the kind referred to above it is usual to vary the brightness of the 7 spot, formed by the cathode ray upon the screen, by applying to the decelerator electrode between the first and second accelerator electrodes a varying potential which is negative with respect to the cathode. The decelerator may therefore be regarded as the modulator. The electrodes are commonly inthe form of metal tubes having apertured diaphragms, and it is usual to make these diaphragms in the form of planar metal discs, provided with circular apertures concentric with the discs.

In an ideal case all the electrons leaving the modulator electrode would reach the fluorescent screen, but in practice this is not the case; itis found that they tend to collecton such electrodes as are above the potential of the cathode, and

they are then lost in the form of current in the corresponding electrode circuits. An indication of the efficiency of the tube may be obtained from the ratio of fluorescent screen current to first anode current. This ratio may be increased by lengthening the tubular portion of the modula'tor electrode, but only at the expense of sharpmess of focusing of the beam.

Objects o'fthe present invention are to provide an electric circuit of the kind hereinbefore set forth, in which the efficiency of the tube, measured by the ratio of screen current to anode current, is increased without resultant decrease in the sharpness of focusing of the beam.

According to' the present invention in an electric circuit of the kind hereinbefore specified, at least one of the said decelerators, and/or a cathode shield electrode if provided, comprises an apertured'diaphra'gm' the surface of which facing the screen is of dished or frusto-conicalshape and has its concave side turned towards said screen. H

Preferably the shape of the diaphragm is such that portions" thereof around the aperture are substantially normal to the incident ray, and portions farther from the ray are at progressively decreasing angles to the ray.

The inventien" will now be described with reference to the accompanying diagrammatic drawing in which Figs. 1 and 2 illustrate the electrode system and associated circuits of two already proposed forms of cathode ray tube modified according to the present invention, and Fig. 3 shows' an alternative form of electrode in the tube shown in Fig. 1.

Referringto Fig. I, there is illustrated a cathode ray tube constructed in the manner already described, and having an envelope consisting of a neck portion ZB and'a frusto-conical portion 21, which is shown broken. In the envelope are arranged the acceler'ator l, decelerator or modulator 2 and first anode 3 in the form of metal tubes all of vthe same diameter as the cathode shield 4. f The latter is arranged next to the cathode" 5', the other electrodes being arranged in the order mentioned proceeding from the cathode shield 4. The accelerator tube I lsshort and has, an apertured diaphragm 6 in the end remote from the cathode 5. r The modulator tube 2 is longer thanthe accelerator and has an apertured diaphragm intermediate between the ends of thetube and on the cathode side of the centre ofv the tube. The; first anode 3: is of much greater length than the modulator, and has an apertured diaphragm 9 in the end nearer the cathode and a second diaphragm III with a larger anode 3 there may be provided two pairs of defiecting plates for causing the ray to scan the screen. One pair of these plates are shown at 22 and 23 and the other indicated at 24. Alternatively, magnetic deflecting coils placed outside the envelope of the tube may be used.

Connections are taken from the various electrodes to tapping points on a potentiometer l1, l8, across which a difference of potential is maintained by means of a source of directcurrent I9. The end H of the potentiometer and the terminal 20 of the source of potential are earthed. It is arranged that the end I8 of the potentiometer is made the positive end. The cathode 5 is connected to a point 22 on the potentiometer which is slightly positive with respect to earth. The modulator 2 may then be said to be biased slightly negatively relative to the cathode; The amount of this bias may be of the order of 50 volts. Modulating potentials are applied to the modulator 2, for example from a radio receiver 2 I, and it is arranged that the modulating potentials are in the negative direction relative to the bias potential of the modulator 2.

A tube as described above gives a well focused beam of electrons on the fluorescent screen 25 disposed on the side of the second anode 8 remote from the cathode in 'the following way. Electrons are accelerated from the cathode 5 by means of a relatively low positive potential on the accelerator I; after passing through the accelerator I the electrons are decelerated to the modulating electrode 2 which is at or near zero potential.

with respect to the cathode screen 5. The high positive potential of the first anode 3 then accelerates the electrons from the modulator aperture, and they pass through the two apertures in the first anode 3 which is held at a higher positivepotential than'the accelerator I. The second anode 8 is maintained at a potential higher than that on the first anode 3, and the field between these two anodes forms an electrostatic lens which focuses the electrons into a fine spot on the screen 25. v 7

When the diaphragms of the tubes above mentioned are in the form of flat discs with apertures in their centres, it is found that the average ratio of screen current to first anode current is less than unity. I

As already stated, it is possible to decrease the first anode current and consequently increase the second anode current or screen current by simply lengthening. the modulator tube on the first anode side of the modulator aperture. This is accomplished, however, only at the expense of the focus which becomes very poor.

According to the present invention, however, the eificiency can be increased by providing a modulator diaphragm'the surface of which has a dished or frusto-conical shape, positioned in such a way that the aperture in it is nearer to the cathode than the rim where it joins the modulator tube. The effect of such a tube is to so shape the equipotential lines of force that they tend to converge the electrons into a beam which passes through the apertures of the first anode, so that only a very small fraction of the beam is collected by the first anode.

The preferred shape for the aforementioned dish-shaped diaphragm is hemispherical with its concave side turned towards the screen end of the tube, but the invention is notrestricted to this shape. The, diaphragm may for example be parabolic. In Fig. 1 the modulator diaphragm 1 is shown as having a surface II of hemispherical shape, and having its concave side turned towards the screen.

In Fig.2 there is shown a cathode ray tube having a cathode 5, a cathode shield 4, a first accelerator I2, a decelerator and modulator I3, a second accelerator I4, a second decelerator I5 and first and second anodes 3 and 8. All the electrodes are in the form of co-axial metal tubes of equal diameters with the exception of the second anode B which has the form. already described with reference to Fig. 1.

These electrodes are arranged within an evacuated envelope as shown in Fig. 1. The electrodes are connected to tapping points on the potentiometer II, I8, in such a way that the first and second accelerators I2 and I4 are given the same positive potential relatively to the cathode, and the second decelerator I5 a potential a few volts positive relatively to the cathode. Modulating potentials are applied to the modulator I3 in the manner indicated in Fig. 1.

In this tube, according to the present invention there is provided a dish-shaped surface II on the diaphragm 1 of the second decelerator l5, shown in this case as hemispherical, and with its concave side turned towards the fluorescent screen.

In operation of the tube the electrodes of the tube are maintained at various voltages of which the following is an example. The potentials are given relative to the potential of the cathode 5. The cathode shield 4, volts; first and second accelerators I2 and I4, +250 volts; second decelerator I5, +'volts, first anode, +800 volts; second anode +4000 volts. The intensity of the beam is modulated by varying the potential of the modulator I3 from zero to about --30 volts.

If desired, the first decelerator I3 may be provided with a dished diaphragm having its concave side turned towards the fluorescent screen. A cathode ray tube according to the present invention, of the kind shown in Fig. 2, may have either or both decelerators provided with such diaphragms.

An advantageous focusing eifect is also obtained if the cathode shield 4 of either of the tubes shown in the drawing has a diaphragm of dished form with its concave side turned towards the screen. Such a diaphragm serves to concentrate more electrons through the aperture in the first accelerator (I, in Fig. 1 or I2, in Fig. 2).

Though hemispherical diaphragms have been shown in the figures, other dished shape or the like diaphragms may be used. For example, the surface II may be frusto-conical, or parabolic. shown in figures, other dished shape or the It is preferred to use surfaces such that portions thereof around the aperture are substantially normal to the ray, and portions farther from the ray are at progressively decreasing angles to the ray. The surface has, in other words, preferably the form of the surface of revolution of a curve.

It is not essential for the dished or frustoconical diaphragm according to the present invention to present a continuous surface. The diaphragm may thus comprise a plurality of flat plates spaced at short intervals apart and having co-axial apertures of progressively varying diameters. Thus the modulator 2 of Fig. 1 may be replaced by the electrode shown in Fig. 3 where the dished diaphragm is composed of the four fiat apertured plates 1, Ha, llb and He.

The aperture in the diaphragm of the modulator electrode may be lengthened in any of the ways, and for the purposes, described in patent.

application Serial No. 50,121.

I claim:

1. An electric circuit comprising a cathode ray tube of the hard type, having arranged within its envelope in the order mentioned and in register with each other, a cathode, a first accelerator, a decelerator having an apertured diaphragm, second accelerator and a screen, the apertured diaphragm in said decelerator having a surface of dished shape with its concave side turned towards said screen, and, means for applyingto said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

2. An electric circuit as claimed in claim 1, wherein said diaphragm is of frusto-conical shape.

3. An electric circuit as claimed in claim 1, wherein said diaphragm has the form of a surface of revolution of a parabola.

4. An electric circuit comprising a cathode ray tube of the hard type having an evacuated envelope within which are arranged in the order mentioned and in register with each other a cathode, a first accelerator, a second accelerator and a fluorescent screen, and between said cathode and said first accelerator a cathode shield electrode and between said accelerator and said second accelerator a decelerator electrode, one of said electrodes having an apertured diaphragm the surface of which facing said screen is of dished shape with its concave side turned towards said screen, means for applying to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

5. An electric circuit comprising a cathode ray tube of the hard type, having arranged within its envelope in the order mentioned and in register with each other, a cathode, a first accelerator, a decelerator having an apertured diaphragm, second accelerator and a screen, said cathode emitting electrons constituting the rays which reach said screen, the apertured diaphragm in said decelerator having a surface of dished shape with its concave side turned towards said screen, said surface around said aperture being substantially normal to the ray and portions farther from the ray being at progressively decreasing angles to the ray, and means for applying to said accelerators potentials positive with respect to the cathode potential and tosaid decelerator a potential approximately equal to the potential of said cathode:

6. An electric circuit comprising a cathode ray tube of the hard type, having arranged within its envelope in the order mentioned and in register with each other, a cathode, a first accelerator, a decelerator having an apertured diaphragm, second accelerator and a screen, the apertured diaphragm in said decelerator having a surface of part-spherical shape with its concave side turned towards said screen, and means for applying to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

'7. An electric circuit as claimed in claim 6, wherein said decelerator has a hemispherical surface.

8. An electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope in the order mentioned and in register with each other a cathode, a cathode shield, a first accelerator, a decelerator, a second accelerator and a screen, said decelerator electrode having an apertured diaphragm the surface of which is of dished shape and having its concave side turned towards said screen, and means for applying to said cathode shield a potential approximately equal to the cathode potential, to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

9. An electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope in the order mentioned and in register with each other a cathode, a cathode shield, a first accelerator, a decelerator, a second accelerator and a screen, said decelerator electrode having an apertured diaphragm the surface of which is of hemispherical shape and having its concave side turned towards said screen, and means for applying to said cathode shield a potential approximately equal to the cathode potential, to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

10. An electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope in. the order mentioned and in register with each other a cathode, a cathode shield, a first accelerator, at decelerator, a second accelerator and a screen, said cathode shield having an apertured diaphragm the surface of which is of hemispherical shape and having its concave side turned towards said screen, and means for applying to said cathode shield a potential approximately equal to the cathode potential, to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

11. An electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope in the order mentioned and in register with each other, a cathode, a first accelerator, a first decelerator, a second accelerator, a second decelerator, an anode and a screen, one of said decelerato-rs being provided with a diaphragm of dished shape having its concave side turned towards said screen, and means for applying to said accelerators potentials positive with respect to the potential of said cathode and to the decelerators potentials approximately equal to the potential of said cathode.

12. An electric circuit as claimed in claim 11, wherein said first decelerator is provided with said apertured diaphragm of dished shape.

13. An electric circuit as claimed in claim 11, wherein said second decelerator is provided with said diaphragm of dished shape.

14. An electric circuit comprising a cathode ray tube of the hard type, having arranged within its envelope in the order mentioned and in register with each other, a cathode, a first accelerator, a decelerator having an apertured diaphragm, second accelerator and a screen, the apertured diaphragm in said decelerator having a surface of dished shape with its concave side turned towards said screen, and means for applying'to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential which is within a range which is substantially negative with respect to the cathode potential.

15. An electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope a screen and, in the order mentioned and in register with each other, electrodes acting respectively as a cathode, a cathode shield, an accelerator, a decelerator, a first anode and a second anode, said decelerator having an apertured diaphragm of dished shape with its concave side facing said screen, means for applying to said accelerator and said anodes potentials which are positive with respect to the cathode potential, and to the cathode shield a potential substantially equal to the cathode potential, and means for maintaining the potential of the decelerator within a range which is substantially negative with respect to the cathode potential.

16. An electric circuit comprising a cathode ray tube of the hard type having arranged within its envelope a screen and in the order mentioned and in register with each other electrodes acting respectively as a cathode, a cathode shield, an accelerator, a decelerator, a first anode and a second anode, said cathode shield having, an apertured diaphragm of dished shape with its concave side facing said screen, means for applying to said accelerator and said anodes potentials which are positive with respect to the cathode potential, and to the cathode shield a potential substantially equal to the cathode potential, and means for maintaining the potential of the decelerator within a. range which is substantially negative with respect to the cathode potential.

17. A circuit as claimed in claim 16, wherein said surface is of part spherical shape.

18. A circuit as claimed in claim 16, wherein said surface is frusto-conical.

19. An electric circuit comprising a cathode ray tube of the hard type having an evacuated envelope within which are-arranged in the order mentioned and in register with each other a cathode, a first accelerator, a second accelerator and a fluorescent screen, and between said cathode and. said first accelerator a cathode shield electrode and between said accelerator and said second accelerator, a decelerator electrode, one of said electrodes having an apertured diaphragm of dished shape simulated by a plurality of annular diaphragms, said surface having its concave side turned towards said screen, means for applying to said accelerators potentials positive with respect to the cathode potential and to said decelerator a potential approximately equal to the potential of said cathode.

20'. A cathode ray tube comprising an envelope housing in which is included an electron source, an electrostatic shield surrounding said source, a series of accelerating and decelerating electrodes alternately positioned in register with and longitudinally spaced from said source, an anode having a plurality of apertures aligned along the longitudinal axis of the tube and in register with said electron source,'a second anodehaving a diameter greater than that of the first diameter axially aligned with said first anode and longitudinally spaced" therefrom and from said electron source, and-deflecting plates partially enclosed by both the first and second anode.

FREDERICK HERMES NICOLL. 

